The concentrations of radioactive isotopes of elements such as potassium, uranium, and thorium (K-U-T respectively) in subsurface earth formations provide valuable geophysical and petrophysical information regarding subsurface earth formations.
One method for obtaining such concentrations is known particularly as gamma ray energy spectroscopy logging. In accordance with such a method, energy spectra of gamma rays from the formation are derived as a function of borehole depth by means of a logging instrument suspended within a borehole and made to traverse the formations of interest.
More particularly, as the logging tool is raised up the borehole apparatus within the instrument detects gamma photons from the formation incident thereupon and measures the corresponding energies of such photons. A plurality of composite total observed energy spectra are thence electonically defined. Each spectrum corresponds to a different increment of or location along the borehole. Each spectrum is comprised of count rates of such gamma photons detected within each of a plurality of successively adjacent discrete energy ranges or windows while the instrument is positioned at or traverses the particular borehole increment.
Each radioactive element distributed in the formation will give rise to and be associated with a unique elemental gamma ray spectrum. The spectral shape and magnitude of each element's spectrum measured in the tool is dependent upon the concentration of the particular element in the formation and borehole variables including borehole diameter, mud weight, diameter and weight of casing, and density of material in the casing-borehole annulus with respect to cased boreholes. Each composite observed total gamma ray energy spectrum may be considered as a linear combination of such individual elemental energy spectra for the different radioactive elements in the formation.
Test formations with known K-U-T elemental concentrations have been employed to generate a plurality of reference elemental energy spectra. Each such reference spectrum has a characteristic shape and magnitude corresponding to a known concentration of the element and standard borehole conditions. Techniques, also based on test pit data, have further been developed for modifying these reference energy spectra as a function of the particular non-standard borehole conditions encountered in the field.
More particularly, the attentuating and downscattering effects of the particular borehole conditioins on the detected gamma ray energies is modeled by means of a borehole compensation variable known in the literature as L. The variable, being related to the density and thickness of borehole materials, will accordingly reflect the amount and character of the borehole materials between the elemental sources of radioactivity in the formation and the gamma rays detected in the tool.
By several methods known in the art (such as a weighted least squares fitting algorithm), various combinations of these borehole compensated reference spectra are selected to minimize the fit error between the combinations of elemental spectra attempted and the composite total observed energy spectrum or gamma ray log measured by the logging instrument. It may be inferred that the elemental concentrations corresponding to each reference spectrum of the particular combination with minimizes the error represent the elemental concentrations in the formation being measured.
It will be recalled that the borehole compensation variable serves to model the effects of borehole materials between the radioactive source in the formation and the detector in the tool through which the detected gamma rays must pass. The variable employed assumes a similar uniform distribution of all the naturally radioactive elements in the formation. It is assumed that all measured gamma rays must travel through and accordingly be attenuated by the same borehole materials, and thus the same borehole compensation variable operates on all elementa reference spectra.
A more detailed discussion of the previously described technique and principles of multi-function compensated spectral gamma ray logging systems may be found in U.S. Pat. No. 4,527,058 entitled "Earth Formation Density Measurement from Natural Gamma Ray Spectral Logs"; "Applications of the Compensated Spectral Natural Gamma Tool" by Gadeken, et al, Transactions of SPWLA 25th Annual Logging Symposium (June 10-13, 1984); and "A Multi-function Compensated Spectral Natural Gamma Ray Logging System", by Smith et al, SPE Paper No. 12050 presented at the 58th Annual Technical Conference, San Francisco, Cal. October, 1983, such references being herein incorporated by reference.
The assumption of substantially homogeneously distributed radioactive sources in earth formations is normally justified in practice. However, situations arise wherein non-uniform source distributions are encountered giving rise to many problems hereinafter enumerated.
As but one example, it is known in the art that subsurface water can serve as a transport medium for uranium or radium salts. Given appropriate flow conditions for such fluid, this can give rise to deposition of radioactive materials in and around the vicinity of the borehole region. More particularly, in practice such material has been found preferentially deposited and concentrated in locations at varying distances radially outwards of and proximate to the borehole center.
For example, such deposits have been found plated in or on formation fractures adjacent the borehole. With respect to cased boreholes, concentrations of radioactive material have further been found on the inside surface of the casing wherein apertures in the casing wall have permitted flow of material (as from corrosion, perforations, or the like).
Also regarding cased boreholes, radioactive elements have further been observed on the outer surface of the casing. Where the casing has been cemented, such depositions or plating are also found radially outwards of the casing at varying locations in the cement annulus between the casing and borehole wall (due to fingering or channeling or the like) and on the outer surface of the cement at the cement-borehole interface.
From the foregoing, it will be appreciated that when deposition of radioactive material occurs non-homogeneously in the borehole region (usually uranium or radium salts), the above-noted borehole attenuation properties operating on the element's spectrum will not be accurately modeled by the conventional borehole compensation variable. Uranium gammas will be less attenuated than those of potassium and thorium, for example, since they will pass through less material. Yet the same borehole compensation variable is being employed in determination of all elemental concentrations.
However, this does not result only in erroneous determination of elemental concentrations of the deposited elemen such as uranium. It will be recalled that elemental concentrations of a plurality of elements such as K, U, and T are determined by combinations of borehole-compensated reference spectra for such elements and that such a combination is sought which minimizes the fit error to the composite spectrum observed. Thus, error in properly modeling and characterizing the contribution of the uranium deposition to the total spectrum gives rise to error in the concentration determinations of the remaining elements such as K and T which also contribute to the observed composite spectrum.
Yet other problems are associated with anomalous radioactive deposition in the vicinity of the borehole. As previously noted, the shape and magnitude of the composite total observed energy spectrum is affected by the amount and type of material through which the detected gamma rays have traveled, whether due to formation lithological properties, casing, cement, and/or borehole material. More particularly, in an energy window of 25-150 Kev, for example, this material causes pronounced in photoelectric absorption of gamma rays and resultant effects the spectrum. The gamma ray count rates of a spectrum in this low energy window region are sensitive as indicators of absorption, particularly by the high atomic number elements in the lithology, casing and the like than are count rates in a high energy window of, for example, 175-325 Kev wherein Compton scattering effects dominate.
Accordingly, photoelectric ratios (i.e. ratios comprised of a count rate in the high and low energy window, such as casing and lithology ratios R.sub.c and R.sub.lith, respectively), as well as their variations with borehoole depth have thus been used as casing thickness and lightology indicators.
Inasmuch as observed spectral shapes are a function of the material between the radioactive source distributed in the formation and the detector, they as well as the photoelectric functions resultant therefrom will thus be a function of the source distance from the detector and the density and atomic number of the intervening material. In conventional determinations and interpretations of the aforementioned photoelectric ratio functions, uniform homogeneous K-U-T source distributions in the formation are assumed.
However, when spectral changes and attendant photoelectric ratio changes are due to changes in the source-detector separation or changes in the intervening material (as, for example, when anomalous radioactive deposition or plating in the borehole region is encountered), this may be erronously interpreted as changes in lithology, casing thickness, or elemental concentrations out in the formation. If, however, it is known that such a deposition situation exists from a number of indicators hereinafter discussed in greater detail, it will be appreciated that these variations in the photoelectric ratios alone, or in combination with other parameters, could be used to indicate whether the source is inside the casing or the radial distance of such depositions from the detector.
This information, in turn, could be used for several purposes also hereinafter discussed. These might include indication of damaged casing or which of a plurality of perforated zones are producing water and/or hydrocarbons (from plating detected on the inner surface of the casing), cement channels, leaks in tubular goods, fractures, and the like.
As an example of prior attempts to detect such depositional activity, total gamma ray count rates from a current log have been compared to those of a historical log run at the same or an adjacent well site. When the current count rates exceeded those of the historical log by more than a predetermined amount (which accounted for statistical variance and variance from normal elemental concentrations out in the formation), then it was assumed that this excess count rate was attributable to radioactive depositions in the vicinity of the borehole.
Such methods did not attempt to compensate for these variations in source distance or position (with respect to the casing) from the detector in determination of elemental concentrations. Nor was there a provision for characterizing the location of these radioactive depositions as being, for example, plated onto the innermost casing surface or the outer casing surface, in cement channels, on the outer surface of the cement, on fractures near the borehole, or combinations thereof.
In summary, then, it will be appreciated from the foregoing that a need has long existed to provide a more accurate determination of elemental concentrations in the formation which accounted for anomalous radioactive depositions in the borehole regions. Still further, however, the need has also long existed for methods and apparatus for determining the presence and location of such anomalous radioactive depositions in the borehole region for various reasons hereinafter discussed.